Rotating knives for cutting wallboard have been inadequately controlled and resulted in "paper pulling". This undersirable paper pulling has required up to 2 inches of each board to be trimmed before sealing with end tape, and, of course, the portion which was trimmed was scrap.
Attempts to overcome paper pulling were first made by installing an eccentric gear arrangement in the motor-to-knife gear train. This arrangement, when driven in a servo configuration with velocity and position feedback taken before the eccentric gears, yielded a desired position/velocity variation profile of the knife operation. The knife profile was synchronized with the board movement so as to reach their relative zero speeds at the 180 degree point of knife rotation. This is also the point of maximum knife penetration when cutting the wallboard.
The above modification corrected the slight error caused when the knife tip (which is synchronized with the moving wallboard) first entered the surface of the wallboard. At this point, the knife tip path was not perpendicular to the plane of the wallboard but rather the knife tip path was at an acute angle with the plane of the wallboard. Therefore, a position/velocity error between the knife tip and the wallboard (in an uncorrected system) was present during the cut, except when the knife tip is at the 6 o'clock, or 180.degree., position. This difference between the knife and wallboard positions caused the paper pulling and the position/velocity correction provided by the eccentric gears eliminated the paper pulling problem.
There is an inherent difficulty in the use of eccentric gearing between the knife and its motor to achieve the variation in the velocity/position profile of the knife necessary to carry out efficient cuts of wallboard sheets which have significant thickness. The gearing must be formed to change the velocity profile of the knife quickly and efficiently before the knife edge reaches the surface target on the board. The problem of the inevitable mechanical wear of the eccentric gearing should be eliminated and a direct control of the knife motor be carried out from an electronic system responsive to board travel under the knife and the knife rotation.